text:
Which is more accurate for height measurement: GPS or barometer?
I am not a pilot and have come to this site to ask a question about the measurement of height.  This is as a result of the comments made as a result of a question in the Physics Stack Exchange forum "Does barometer improve gps accuracy?"
I feel that there will be more of you on this site who have real time experience with flying and so can answer my question which is:
Which is more accurate for height measurement: GPS or barometer?
I have noted that those engaged in geocaching often have GPS units with built-in barometers to improve height accuracy.
One of my friends who is a pilot responded as follows:

A light aircraft relies on a barometer that is set to the standard
  1013 millibars when in cruise at height (or the regional QNH when
  lower) but they set to the local reading for the airfield (QFE) when
  planning their decent. Commercial aircraft also have ground proximity
  radar but they also have a standard set of old fashioned flight
  instruments in case the electronics fail. My old aircraft GPS did
  indeed read out height as well but you were told never to rely on it!

(Not relevant to the actual question, just a note on your friend's comment) I think pretty much only gliders use QFE (Q-codes suck by the way. Why can't non-US aviation learn to use natural language?), iirc it's the setting that makes your altimeter show zero at ground level for the particular airport. It's easy to set when you're on the ground (just twist until it reads zero), and gliders often return to the same airport. Most other aircraft use the QNH (also known as altimeter setting) for the region and simply add the ground elevation to whatever AGL altitude they're shooting for.

@falstro Thanks for your comment.  Does it hint at the fact that I should be flying with my friend in his light aercraft? :-)

Usually we measure altitude. A barometer provides only a barometric altitude based on pressure so the actual altitude may vary. On the other hand the differential GPS can provide an altitude with a millimeter precision, but it takes time. Altitude/height concept is complex (image taken here)

Your precise height above the ground is not nearly as important as that everyone agrees on the height.   Even if one value is more accurate than another, everyone within an airspace should use the same measurement technique.

Is radar altimetry uncommon enough that it's not worth asking about in this question?

In order to really answer your question, I believe that you need to more clearly define height for us: height above the ground (AGL), Height above mean sea level (MSL), height above the WGS84 ellipsoid, the earth geodic model (EGM96), or some other standard?

@Lnafziger As a non-aviator, that may not be a distinction the original poster is even aware of.

@Lnafziger I have heard of some of the different definitions of height. For me the height that matters is the one that makes sure that the pilot does not unknowingly hit the ground.or any other aircraft.

Directly comparing accuracy of GPS, barometric altimeter and radio altimeter makes little sense, because they each measure different quantity.

GPS measures ‘geometric altitude’. This is the actual height above the reference ellipsoid, in units of length. It is fairly accurate, but there are external reasons it might be unavailable.

Barometric altimeter measures ‘pressure altitude’. It is given units of length, but it really is just a barometric pressure, expressed in funny scale. It only corresponds to altitude when the atmospheric conditions match the “International Standard Atmosphere,” which they of course usually don't.
The biggest error is due to pressure, which might be corrected using the altimeter setting. With this, the altimeter reading is better approximation of actual altitude, but since temperature also changes the rate of pressure change with altitude, it still only matches the geometric altitude exactly on the ground at the airport for which the altimeter setting is.

Radio altimeter measures ‘height’ above ground level.

Now in aircraft, each of these quantities has different use:

The geometric altitude from GPS is, together with a topographic map, used in EGPWS to alert pilots that they are flying too low. Compared to older GPWS that only used the radio altimeter this has the advantage that warning can be given when the aircraft is high enough above ground directly below, but approaching higher ground where it won't be.
GPS is good for this, because when it works, it always gives the geometric altitude without need for setting that might be incorrect. However, it is a new technology that not all aircraft have and it might be unavailable for external reasons (that can't be mitigated by adding redundancy).

The barometric altimeter is used for aircraft separation. Here, the exact altitude does not matter, what is important is whether the aircraft are at the same altitude or not. Barometric altimeter is excellent for this, because it is old technology, so all aircraft since the early days have it, and it is simple technology, so it is reliable.
Above certain altitude (the ‘transition altitude’, 18,000 ft in the USA, but varies in other parts of the world), all aircraft have the altimeter set to the standard value of 29.92 inHg/1013 hPa. This high, there is no concern of separation from terrain, so everybody just uses the same setting to keep things simpler and errors less likely. The pressure altitude can easily differ from the geometric by couple thousand feet, but nobody cares, because it is only important to know whether the other plane is above, below or same altitude and this serves that purpose well.
Below transition altitude, the altimeter setting from nearest airport is used, so the altimeter reading better approximates the geometric altitude for purpose of separation from terrain. However, the altitudes still only match at the altitude of the airport from which the setting is. Above (and below) the error increases. In cold weather, you can easily be 10% lower height (above ground) than the altimeter says. This is simply handled by adding sufficient margins to the published minimum altitudes.

The height measured by radio altimeter is used in the GPWS and EGPWS systems to warn pilot of flying too close to the ground and for announcing the height during final approach so the pilot can better judge the landing without having to look at the instrument. However, due to terrain irregularities, radio height is not much use outside these two specific cases. Also, radio altimeters usually only indicate up to 1,500 ft–2,500 ft if installed at all (all airliners have it, but most GA planes don't).

The radio altimeter is accurate to a few feet. The barometric altimeter has to be accurate IIRC to 75 ft. Even simple GPS receiver should be able to do that too, but since the values can be easily off by a few thousand feet at high altitudes, they can't be mixed. So barometric altitude is always used for traffic control.

"...if installed at all (most GA planes don't have one)."  If a GA plane doesn't have a radar altimeter, then how does its GPWS work?

@Sean, EGPWS is GPS-based. This has the advantage that it can warn you of high ground ahead, which the radio altimeter can't. Older GA planes usually don't have GPWS either; the newer ones would have it integrated in the avionics suite that has GPS.

Is there a procedure to deal with the transition between two different standards at 18,000ft? Is it possible that two planes are at the same altitude and in danger of colliding, but one pilot thinks they are at 17,900ft based on a locally-calibrated measurement, while another one thinks they are at 18,100ft based on the global standard?

@craq if anybody flew at those levels it would be possible, which is the reason nobody does. A lowest usable flight level is defined that is the lowest altitude above transition (according to standard pressure) that is guaranteed to actually be above transition (according to local pressure) and everybody knows not to fly in between. Also in the USA above 18,000 ft it's class A airspace where everybody must follow instrument flight rules, so they fly whole thousand feet assigned by air traffic control (but in other places the transition may be 5,000 ft and there can be VFR traffic above too).

There is a pretty comprehensive description of the issues at this link.
Briefly - the conversion of pressure to altitude makes certain assumptions about the "standard atmosphere" - that is, it works best when the temperature and the lapse rate (change in temperature with altitude) exactly matches that from the model. In the real world, this is almost never the case. For example, on a hot day the air will be less dense, so the weight of a column of air will be smaller - and so when the pressure seems to have dropped by say 200 HPa, you will underestimate your altitude. The example calculation in the link shows that when the surface temperature on a dry day is 42°C, the error at 10,000 feet could be as much as 800 ft - which is considerably bigger than the typical error in a GPS (although GPS is less accurate in the vertical direction, you should routinely be able to get height within about 50 feet when you have an unobstructed view of the sky, and ionospheric activity is not unusually high). In principle it is possible to make corrections for these things - but a simple mechanical pressure-based altimeter probably doesn't.
On the other hand - if everybody else is flying by barometer, you might be "right" about your GPS based altitude, but "wrong" compared to other planes in the vicinity.
So it is advisable to use the same method as everyone else, so you don't crash into each other. On the other hand, when it comes to your final glide, the GPS is more likely to get you home safely on a hot day.

This depends on the accuracy of the barometer and the weather, since the weather influences the air pressure. So even if you're standing still at the same altitude the barometer will show you different readings. This is why a pilot constantly has to adjust his altimeter. An altimeter is designed to measure the altitude above a fixed level. This is why pilots have to calibrate their altimeters before take-off with ATC.
The GPS system, on the other hand, works with an accurate clock and the known positions of the satellites. The satellite sends out packages which are time stamped. The receiver receives these time-stamped packages and uses its own calibrated clock to determine the sequence. Comparing this to the know positions of the satellite at certain positions allow the use of triangulation formulas to solve for the position of the receiver.
The GPS system is owned by the US military and they have this graph about the accuracy improvements over the years.

So to answer your question, it is determined by the equipment you use, but cost wise it is cheaper to get an accurate reading from a GPS since it is also less influenced by the weather.

"The receiver [...] uses it's own calibrated clock to determine the sequence.": The receiver would need to be fixed and use an atomic clock for this to work. Actually 1/ the receiver clock is not precise and 2/ its speed is unknown. Pseudo-range name is used for that reason. It evaluates what could logically be its local time from at least frames from 4 satellites and by successive approximations guesses the offset between its local clock and the GPS clocks. Elegance of the GPS system is only the satellites need to have a precise clock.

Interesting - the graph doesn't indicate if it's showing error range for horizontal location (lat/long), vertical location (altitude), or both. I'd be careful assuming that this error range also includes altitude, as GPS "accuracy" is usually (in a non-aviation sense) perceived to mean surface location.

@FreeMan. Good catch as it is none of them. But the embedded frame explains SIS URE. It's an error in the pseudo-range, due to satellite data itself. This assumes a perfect receiver in perfect view conditions (DOP), and no ionospheric perturbation. Said otherwise it's an assessment of the "space segment" only.

In my experience with actual GPS units, altitude data are quite erratic and at least appear to be less accurate than lateral measurements.

@DavidK: It's normal vertical data is less accurate. To be equivalent the receiver would need to have access to satellites at say 45° below the horizon (mentally exchange height and latitude of an aircraft and see how satellites now help in determining height, but are less efficient to determine the horizontal location).

@mins Thanks, it's good to know there is a theoretical reason for this, rather than merely some defect in my particular receivers.

From the point of view of a particular pilot, is the accuracy of the GPS system not related to the quality of that pilot's receiver? I guess more to the point - are there receivers and/or places of poor satellite coverage that would make barometric altitude a better choice?

@ToddWilcox, you need at least 3 satellites to get a position, the more satellites you have the more accurate you position becomes in the 3D space. There might be spots on the earth that are not covered by GPS but that must be at the ocean since almost all the landmass is covered. The barometer only works with great accuracy if you can set it to a known height, like at the start of a hike.

@Brilsmurfffje: There are no areas not covered with the GPS satellite constellation. "Covered" means that 95% of the time there are 4 active satellites in sight and average SIS URE is less or equal to 7.8m. For this at least 24 satellites are active on carefully selected 12-hour orbits. See this for more details. Remember the GPS was built to provide navigation means to NATO cruise missiles, ships and aircraft, in addition of land operations).

Most likely GPS is more accurate, after all, it can calculate your location in three-dimensional space within a few feet. However, it's not really that usable, since GPS measures altitude from the geoid. That said, new GPS-receivers should be able to correct the difference between the geoid and actual earth surface.
Baro-altimeters are also quite accurate, but only when the atmosphere is at ISA-conditions. Baro-altimeters are calibrated to ISA-atmosphere, and when any condition varies from this, you get an error. Some of those, for example temperature error, can be compensated for, but removing all potential errors is a tedious task.
If I was geocaching and wanted the most accurate elevation information for my location, I would use an old-fashioned geographic map with elevation information.

Just a comment: The GPS determines the altitude on the ellipsoid. Then using a table of geoid undulations it approximates the altitude (and vertical direction) on the geoid. The actual AGL cannot be determined without using the equivalence of a detailed map (which the GPS device may have or not). See this for more details.

@mins thanks for info. I had it backwards then. Relying on nav-lessons from flight school..

Do you have anything to back up the claim of the 3D accuracy? I was under the impression that satellites are more overhead than sideways, thus I would guess the lateral accuracy is higher than the vertical one.

@falstro Nothing solid. Just the fact that most modern receivers use 6, 8 or even over 10 satellites to determine it's position. This increases the accuracy, and diminishes, to some extent, the effect of satellite positioning. But you're right, vertical accuracy is smaller.

Ok, so what makes you say that it's "most likely" the GPS? My experience is to the contrary with the GPS being off by hundreds of feet, but I don't have any real evidence.

VDOP vs HDOP definition and example.

@falstro: Determination of oceans height is done using GPS (from another satellite, like Topex-Poseidon, and radar) and reaches 1 cm precision. Modern geodesy is done by GPS, and maps are accurate.

@falstro Like I said, the baro-altimeter has several errors. To get accurate altitude from a baro-altimeter, you should know the exact atmospheric pressure, the temperature gradient and even compensate for the wind. The GPS doesn't suffer from these errors.

+1 for the use of topo maps! I question the accuracy of GPS in determining altitude. I realize the GPS chip in my phone is at the (very) cheaper end of the scale, but when it shows different altitudes at the beginning and end of my run when I've started and stopped the tracking right in front of my house, I begin to lose faith...

@FreeMan: GPS receivers have the inconvenient of not working well under foliage, and need to have a good view on 4 satellites disposed in a good configuration (not grouped). That means also be cautious not to block the antenna (which may be buried in the device box). If you take care of these conditions, and get a fix during 5 minutes, then you must have good results. SiRF Star III + chips help to speed up the fix. You must have allowed WAAS/EGNOS (differential GNSS) else the precision is a mere 30m..

Good point, @mins, I have an app that shows all the satellites I'm viewing at the moment, I'll have to fire it up before and after my next run to see what it's showing. Usually, I seem to see 8-10 at any one time around my house.

@FreeMan: I believe only the "best" 4 satellites are used for a fix. The other are just being received and tuned and ready to replace a selected one if its signal decreases or its position is not as good.

@mins Is GPS mapping comparable to GPS altimetry for aircraft? In other words, doesn't the speed of the aircraft versus the relatively fixed nature of the ground affect the accuracy of real-time GPS altimetry versus GPS mapping? So using GPS mapping accuracy might not be helpful in terms of answering this question?

@ToddWilcox: Actually the receiver may move, the relative speed between a satellite and the fixed point on Earth is already non null, the orbital speed is about 14,000 km/h. In GPS survey the data are recorded and analyzed back to the company offices. This is because surveying is done with the 2 frequencies and some analysis is possible to remove ionospheric refraction and propagation slowdown effects (which affect each frequency differently). It's also possible to determine (or confirm) speed with Doppler shift observation.

To amend already existing answers:
Very close to the ground, GPS altitude is unusable for aviation purposes. Vertical accuracy of GPS position is roughly 1.5 times that of the lateral position. Unaugmented GPS signal might have position of over 10 meters, giving even larger vertical error. For that reason, vertical GPS position is not used for approaches if some augmentation method is not available. Even then, GPS altitude is not used to determine approach minima, which is defined as barometric altitude.
There is also problem of momentary degradation of GPS accuracy due to satellite constellation or interference from building, terrain, weather, jamming etc. which would be unacceptable if the signal would be used for terrain clearance.
During takeoff and landing, barometric altitude is referenced to local air pressure, making it very accurate locally at low altitude. In cruise as mentioned, the accuracy does not matter as much as aircraft maintain suitable clearance from the terrain in the area, so even in case of lateral navigation error there will not be a collision.
The benefits of barometric altitude are separation from other aircraft, as any aircraft arriving from distant corners of world are using same barometric reference of 1013hPa (29.92 inHg) during cruise, and the matter of aircraft performance. Aircraft performance depends on local air pressure and temperature, so fixing the altitude to the barometric reference reduces aircraft performance to a single variable problem, ie. temperature.


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How should I interpret this TAF involving TEMPO and BECMG blocks?
with the reference in the TAF below , I was looking for the VIS at 0500 UTC and I was little bit confused because  of the " TEMPO "
And the TAF was
LHBP TAF (FT)140400Z 141206 34006KT 9999 BKNO45
TEMPO 1215 VRB10G2OKT 7000 SHRA SCT033CB BKNO40
FM1700 CAVOK
BECMG 2022 VRBO1KT 5000 BR NSC
BECMG 2201 1200 BCFG BR SCT002
BECMG 0104 0600 FG OVC001
TEMPO 0406 0150 FG VV001

According to the TAF at 0104Z VIS would be 600M, then we can see TEMPO at 0406 with 150 M VIS, so what VIS can we expect?  Can we say it is 600M with possibility to be 150 M? Or we can say directly it is 150M?

Is this a question for an exam, or an actual TAF that you encountered in reality?

Usually, when we translate TAFs we use the word expect. So, in this case, we expect the visibility to drop from 600m to 150m. So when planning a flight you would use 150m.
Also, I find it odd how the time format is different to what I am used to.
At my local airport, this is the TEMPO in the TAF.
TEMPO 0302/0307 6000 SHRA SCT030TCU

So this would translate.
Between 0200UTC and 0700UTC on the 3rd, we would expect visibility to be 6000m and for there to be rain showers, and towering cumulus scattered at 3000ft.

This is the old-style time format, which the United States switched away from six or eight years ago.

TEMPO means that there were be fluctuations in the weather in the time period specified, lasting as long as 50% of the time range specified. TEMPO 0406 0150 FG VV001 means that in the period between 04:00 and 06:00 UTC the visibility will at some point go down to 150 in heavy fog, and it could last up to an hour in total.
One thing I would point out is that the time range for the TAF ends at 06:00, so its very likely the fog will last longer than that.

So if we got that question with two options , is that 600m with possibility to be 150m and only 150 M which one is the best answer ?

I don't follow you @KomailMohamed

It's going to be both, you will have 600m and 150m during that period of time, and ranges in between. For planning purposes assume the least favorable conditions.

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What is propeller / engine sync and how does it work?
In some models of multi-engine airplanes there is a switch labelled something similar to "prop sync" or "engine sync".
My questions are:

What is engine synchronization?
How is it achieved?
Why is it important to use?
What happens if you don't use it?

Note that this is not limited to propeller aircraft or aircraft with only two engines.  Many multi-engine jets have "engine sync" as well. (Updated question to reflect this.)

Another concern is setting up resonant frequency problem on some airframes.

This question has now received an (at least partial) answer at the new (duplicate?) question https://aviation.stackexchange.com/q/77429/34686

According to Wikipedidea

In aviation, propeller synchronization is a mechanism that automatically synchronizes all propellers of a multi-engine, propeller-driven aircraft so that they rotate at the same speed.

As for why its used the article states

Propeller synchronization serves mainly to increase the comfort of crew and passengers, since its primary purpose is to reduce the “beats” generated by propellers that are turning at slightly different speeds.

As for what happens if you don't use it, according to the article

Synchronization is not normally necessary for proper operation of the aircraft.

just because the props are turning at different speeds, that doesn't mean they're not producing equal thrust. but in reality, no two (or more) engines produce equal thrust. they just need to be close enough not to generate undue yaw. (note that this has nothing to do with beats)

I have edited my answer to reflect this.

I don't know how it is achieved, but here is the reason:
If you have two sources of the same noise, with nearly the same frequency, the sum of both noises will be a noise of slowly increasing and decreasing volume. It's called beat and can become very annoying.
The math says
$$\sin(2\pi f_1t)+\sin(2\pi f_2t)=2\cdot\sin\left(2\pi \frac{f_1+f_2}{2}t\right)\cdot\cos\left(2\pi \frac{f_1-f_2}{2}t\right)$$
To demonstrate this, there you see two sinus-tones of almost same frequency, and what happens if you mix them:

The blue curve is of frequency $(f_1-f_2)/2$.
As example, one machine running at 3000rpm and one at 3030rpm results in a noise which increases for one seconds before it decreases within one second again.
As said, I don't know how it is done, but synchronization must be done very precisely to avoid this beat.
Edit:
Here is what happens if the two noises do not have the same volume. One of the curves has three times the amplitude of the other. The envelope is not a pure sin function, but the blue function fits it quite well.

I think you're missing a $2$ on the RHS of your formula.

That's right, thanks

I have noticed this very thing and thought it was driving me nuts! Glad it wasn't me just going crazy.

One of the curves has three times the amplitude of the other it all depends on the two "volumes", it can be 3 times, it can be 10, it can be anything

This factor 3 was just an example. However, the larger the difference, the smaller the beat effect. The range of the volume is already smaller compared to the max. volume here.

On the Junkers Ju-52 the pilot had two little discs which rotated with the beat frequency between two engines. The upper left one would show the beat frequency between the left and the center engine, and the lower right one that between the center and right engines. The pilot would try to stop them rotating by advancing or retarding the throttles, equalizing engine speed in the process.

I highlighted the synchronization indicator in the picture above (source).
Running all engines at the same speed helps to fly straight and reduces vibrations. Not only is one distinctive engine frequency removed by equalizing it with the other, also the beat frequency is deleted.
Sorry, I cannot answer this for the Beech Baron. I've flown it only once and don't remember any instruments (other than my ears) which would help
with synchronization.

my friend had a twin-screw boat that had a pair of lights side-by-side, each of which was connected to cyl 1 of each distributor. when the lights flashed together, the screws were in sync.

Looks like some of those instruments have had a little update since the WW2 era...

The engines using prop sync will be constant speed props. Constant rpm is maintained with a governor of some kind that adjusts the pitch.
This governor can be tuned by the prop sync to speed up or slow down one prop by adjusting it.

It's just an automatic system to do prop speed and/or blade location synchronization.  On a multi without it, one does it manually by adjusting the prop pitch controls.  If the automatic system is not used, it can still be done manually.  Prop sync is typically required to be turned off during critical phases of flight (takeoff/landing).

4 engine prop planes turn at exactly the same(RPM) or percent depending on the type of engine. Number 3 is usually the boss and all the others slave to it. Every blade has to cycle between the other blades on the same wing or it causes a harmonic vibration so fine if cannot be felt by the crew but can shake nut or other tightened safety devices loose. If 4 blades crossed when they are at their closest proximity, you'd have blades which are near the speed of sound crossing about 4000 times a minute, so they must take turns crossing the center line.
On 2 engined aircraft it is not a critical.

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Does the parking brake provide additional deceleration in an abnormal situation?
During a rejected takeoff at a speed > V1, you obviously try to stop with all available means since a runway overrun is a very undesirable result. In this case, could using the parking brake in addition to the normal brakes and reverse thrust help slow down the aircraft faster?
I know that the parking brake can be used to slow down the aircraft, e.g. in the “no breaks available” memory item on an A320 but does it help when the normal brakes are available?

I don’t believe it helps. Pulling the parking brake would lock the wheels and thus have the aircraft skid on deflated tires and grinding axles. This provides less friction than proper anti-skid rubber-on-asphalt braking and would make matters worse instead better. My hypothesis only, hence comment instead answer.

Wouldn't it be reasonable to assume the parking brake is not much stronger than it needs to be to stop it rolling away from the gate - which is usually on level ground. So it would seem highly unlikely to be able to lock the brakes during the landing roll. Aircraft are very weight challenged - nothing is heavier or bigger than it really needs to be.

Assuming the regular brakes are working, the limiting factor is the friction between the tires and the runway.  The regular brakes should be able to apply enough force to lock the wheels (even if an ABS system prevents them from actually doing so).  So parking brakes would add nothing.

This is probably aircraft dependant, but I can give some examples for the airplanes that I have flown and will be generally applicable to transport category aircraft.  Understand that it won't cover every airplane as an absolute rule though.
In general, these types of airplanes have an antiskid system which keeps the wheels from locking up during heavy braking.  If they were to lock completely up, the tires would likely blow, greatly decreasing deceleration and making it more likely to run off the runway (either at the end or off a side).
Parking brakes often have multiple settings because of this.  One setting will be one which can be used when the normal brakes have failed, and have a relatively low brake pressure / stopping capability compared to the normal brakes in order to protect the tires.  Using this setting will stop the airplane eventually, but you are better off with the normal brakes which have a higher brake pressure with antiskid, and better deceleration.
Another setting would have a much higher pressure, intended to keep the wheels from turning (while it is parked), and this would be undesirable while attempting an abort.
In the end, I will say what I usually say:  Follow the guidance in the AFM.  If it says to use the emergency brake when you abort, then do it.  Most likely it will not (because of the above reasons), and you should not do it.  If you take it upon yourself to go against their guidance, you become a test pilot and are either trying something that hasn't been done before, or worse yet, something that has been tried and has had an undesirable result.


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Could someone please explain why the A320 doesn’t have an anti ice system for its vertical and horizontal stabilisers?
I had come across in the FCOM that there is no anti ice system for the vertical and horizontal stabilisers while looking at the schematic diagram. I find it very strange that the other surfaces on the empennage do not have anti-ice systems.

On PPrune I found:

The aircraft has been designed to account for that. (Simplistically,
  the tail is over-sized to compensate for reduced effectiveness when
  ice forms; it's easier/cheaper/simpler to make the tail a bit bigger
  and take a weight penalty than to install a whole extra anti-icing
  system on the tail, which is why that's the typical approach for
  higher speed aircraft)

To me this makes sense, as the weight penalty for making the tail a bit larger is not as big as the weight penalty for making the wing a bit larger.
Another thing that is different between wings and empennage is ice build up on the wing near the engine could suddenly let go and end up in the engine and cause damage. To prevent the ice buildup, anti icing is used. On the empennage this is no issue.

So this is basically saying that the weight penalty for an additional anti-ice system, with all of what that entails, is larger than the weight penalty for making the stabilizers a bit larger to compensate for potentially reduced efficiency?

Yes, that's how I read it

The vertical stabilizer/rudder is already oversized for all but a full-thrust takeoff, where it would have been properly deiced to begin with.


text:
How are off-wing jet engines transported?
One of the optional services in Rolls-Royce TotalCare is "Engine Transportation"

This service includes routine transportation of engines between the customer’s main base (or designated station) and the engine shop, and/or remote site transportation.

Jet engines can be very large heavy and expensive items. How are they transported? Are they flown or do they use road transport? Are there dedicated vehicles?

With difficulty.

They have the usual transport options: shove into cargo area, on a flatbed truck, shipping container,...
They can also be attached below the wing of a large aircraft and flown with if there is no room in the cargo area.
(stolen from this question)
Notice the extra engine on the near wing.

To add to this answer, jet engines are not all that heavy relative to other bulk items but large and cumbersome.  It is a pretty common even to see engines in harnesses, covered in tarps being transported by flatbeds.

The Conroy CL-44 "Skymonster" was built for the purpose of transporting three Rolls-Royce RB-211 engines. It was a derivative of the Canadair CL-44 and was meant to transport the engines for the Lockheed Tristar. Jack Conroy had before modified old C-97s to Super Guppy transporters, and those initially transported Saturn rocket stages for NASA.
The picture below was shamelessly copied from the Wikipedia article. Note the bulges near the tail: They cover the hinges for the sideways opening fuselage.

Today, Boeing 747 freighters can swallow most engines, so in the age of widebody transporters there is no more need for special aircraft for engine transport. For the ones that don't fit inside the 747, there is always the Antonov An-124.

Having lived for nearly 15 years around the Boeing factories in Seattle, it was a common sight to see a flat-bed truck with a tarp-covered engine driving up and down the freeway.
Just like other big industrial machinery, they build a mounting for it, put it on a truck, and drive it down I-5.

Jet engine transportation is a highly specialized task, the primary method is ground transportation via truck and trailer.
It is important that the transport company uses fully air-ride trailers and advanced load securement techniques to ensure zero stress on sensitive engine components.
Peter Kämpf - While you are correct that some engines can fit inside of an aircraft for transport, the cost for this service is enormous when you compare it to a truck and trailer. Also larger engines such as the Trent 1000 (Fits on a 787 Dreamliner) will not fit inside of an air cargo carrier.

If you consider the capital cost for an engine, financing ownership for the duration of truck transport will exceed air transport cost on longer distances. And trucks can't cross oceans, but need to be loaded on a ship. Engines don't go in standard containers, but on palettes. Then they will fit comfortably inside a 747 Freighter.

It's apparently pretty hard to get outer engine dimensions, but based on fan size and adding a bit, all but the largest engines will go on a standard truck. Oversize permits are readily available.
Rolls-Royce makes engines in Derby (amongst other places) but not at the airport. ( East Midlands airport has one runway, 8200 feet, which is a bit tight for a 747.) Therefore, engines must leave the factory by truck at least once.
I expect that the decision to send the dud engine for service by truck, boat, on the wing or in the hold will be purely economics.

Given engines are bulky but light, I'd guess any widebody shipping an engine would be well under MTOW, and hence have no issue at East Midlands. I've not checked the weight beyond "not heavy" though.


text:
What is the difference between flaps and slats?
I have heard pilots talk about flaps and slats, seemingly interchangeably.
Is there a difference between a flap and a slat or they are the same thing?

Flaps are at the back of the wing, slats are at the front.

Easy! :)
If you want it more technical: They both help to generate more lift. See these diagrams with angle of attack and lift coefficient:

As the lift coefficient is inversely proportional to the minimum airspeed, a higher CL will allow a lower Vmin.

Both the lectures that I know and almost all sources on the internet describe a reduced stall AoA with flaps, i.e. a c_L / alpha curve that is moved both up and towards the left. This is important, because it means that an extension of the flaps alone can cause a stall without changing the AoA.
Forum post with good sources: http://forums.x-plane.org/?showtopic=59032

Just wanting to point that flaps can be located both at front and back of the wing (trailing-edge or leading-edge flaps), while slats only exists at the front. Main difference between slats/flaps is not location ,but (as you pointed out) functionality

How the drag coefficient affected by the slats and flaps?

Slats and flaps both change the shape of the wing when they are extended and allow the wing to generate more lift so that the airplane can fly slower.
Operationally they are both retracted and flush against the wing except for takeoff and landing at which time they are extended.
Pilots tend to refer to them together because they are used for the same purpose and used at the same time.  In fact, they are normally even moved by using the same control (typically the slats comes out when selecting first detent, and then the flaps come out progressively further at each detent):

Slats are on the front of the wing (on the left in this picture) and the flaps on on the back of the wing (on the right).  As you can see, they both tend to make the wing bigger and more curved when they are extended:

It's worth noting that there are also autoslats, which extend automatically at high angle-of-attack, so it's not quite true that slats are only used at take-off and landing.

@DanHulme "Operationally" that is the only time that they are used.  My airplane also has auto slats, but they only extend during non-standard events (flying too slowly without them extended), and I don't feel that the autoslat feature adds any to the discussion of what they are for.  :)

That's fair enough. I just think alpha protection is a good enough use to merit a mention, even if only as an aside, as it's something you can't reasonably do with flaps.


text:
(How) do pilots round the time?
I'm wondering what's the time for pilots.
Given the case the watch says 10:00:20 - is it basically 10:00?
And when does 10:01 start? Is it past 10:00:30 or only as the seconds move from 10:00:59 to 10:01:00?

Welcome to the site @Qohelet. Rounding may or may not be appropriate, depending on the context. Do you mean for recording a log, or making a turn, or some other purpose?

@GdD - thank you for your quick answer. I'm pretty new to aviation, so I'm not aware if and which differences exist depending on the context. I was mostly thinking of airport-related cases - for instance when you get instructions from the ATC.

Instructions from ATC are rarely time dependent, and when they are it's to the minute, not the second. I'm sure there's a definition somewhere but realistically it's rarely important.

@Qohelet If you don't agree with my edit, please rollback. I think my edit is what you meant.

For what it's worth, personally, I always just truncated the seconds if they were shown. For position reporting and the updating of ETAs we were allowed a 3 minute window, so the seconds weren't that relevant.

The roundings I know of in the context of US general aviation aren't strictly defined or enforced because they don't need to be (with the exception of racing; see below). The military might need to be more precise, but I have no idea about that.
Anyway, here are the main roundings a typical GA pilot might encounter:
Logging: to the nearest 6 minutes
Pilots usually log their flight time in tenths of an hour, i.e. they round to the nearest 6 minutes. There's no regulation on it and some people prefer to log in hours and minutes (maybe it's even required in some countries), but tenths seems to be most common.
ATC instructions and clearances: to the nearest minute
See this question for information on timing under IFR (some of it applies to VFR too); instructions, clearances and reports are given to the nearest minute. It is required to have a clock with a second hand but that's to make it possible to do timed turns, not because ATC gives instructions down to the second.
Air racing: to the second or less
In air racing, there are at least two ways of using time. In a speed race - like the Red Bull Air Race - you measure the elapsed time over the course. The timing for Red Bull is to one thousandth of a second.
In precision navigation air racing, you plan your flight over a specific course and calculate the time (and sometimes even the fuel) you'll require to fly it. When you fly the course, any deviation from the numbers you submitted beforehand - under or over - is penalized. Timing in those competitions is to the second.

I never think about this as there's no use cases I can think of where accuracy to the second is required in navigation or communications. ATC clearances are either not time based or are in a time block. Trying to use exact time in ATC communications would just cause confusion because not everyone's watch or clock is synchronized. Imagine if ATC told me to line up on a runway at 10:21:30, but my watch is 20 seconds fast, I could line up on a runway just as an airplane is landing!
Most of ATC communications is situational: do this now, or do this after another event; "Line up on runway 22 left", "report at point X", "cleared for takeoff". A departure clearance would read like "After departure maintain runway heading to 3000 feet, then turn left to 270 and climb to 8000 feet." There's no time mentioned, the clearance is valid from the moment received and is good until it's completed or revoked.
A flight plan is valid for a certain period, usually before and after the planned takeoff time as HH:MM. Seconds are not recorded.
If I record flight time in a logbook I don't even look at the second hand really, I generally look at the minute shown and use that. If it's past 30 seconds and I notice I may round up to the next minute, or I might not, it doesn't really matter. If I saw an event that I thought was important, like an incident, I would write down the exact time in seconds as that level of detail may be useful.


text:
Which types of aircraft are required to carry ELTs?
According to regulations and recommendations from the FAA, ICAO or other aviation organisations, what types of aircraft are required to carry Automatic Fixed Emergency Location Transmitters (ELT(AF)) devices? In particular, is it required of commercial transport airliners?
CAA (and I guess other aviation organisations/regulators) group aircraft into several categories:

General Aviation (GA) - personal aircraft etc.
Business Aviation - executive jets etc
Commercial Air Transport / Public Transport - Scheduled passenger airliners etc.

AOPA say

ELTs are emergency transmitters that are carried aboard most general aviation aircraft in the U.S.

(my emphasis) - note GA, not all aircraft.

CAA refer to ICAO Annex 6

Aeroplanes - ICAO Annex 6, Part II
  6.12.3 From 1 January 2005, all aeroplanes operated on extended flights over water as described in
  6.3.3 b) and when operated on flights over designated land areas as described in 6.4 shall be
  equipped with one automatic ELT. [i.e. designated KK(ii) in the Air Navigation Order]
6.12.4 Recommendation. - All aeroplanes should carry an automatic ELT.

But I can't find an annex mentioning ELTs at www.icao.int.

So, I'm not sure whether all airliners are required to carry ELT(AF)s and if so, by whom.

In the quoted text from AOPA I believe the term "general aviation" is being used in the sense of "all regular aircraft" and it was not intended to exclude large commercial aircraft from that category.

@Tyler: I wondered about that but an AOPA glossary and another suggests GA excludes airliners.

Well, in the context of an airport ramp it kind of does. At large airports they have various "terminals" and then a separate "general aviation" terminal and sometimes a "cargo terminal", the general aviation being whatever is not big body major carrier or cargo. However, even here the meaning is ANYBODY can use this terminal. Re-interpreting this term to mean non-major-carrier has just happened sort of randomly because major carriers do not use the GA terminals at large airports. So, for example charters are "GA" even though they are commercial carriers. It's confusing.

@Tyler: It probably doesn't affect answers to this Q so I've asked a separate question

Everything you need to know (at least in the USA) is here.
Regarding airliners, it appears that they are carved out of the general requirement to have an ELT in 91.207.f.2.   I guess that makes some sense.  If you're flying a scheduled flight, presumably we should be able to find the plane, since we know the takeoff time and approximate airspeed, and we have radar tracking by ATC/ARTCC.  Unscheduled (especially VFR) flights are where things get dicey.


text:
What is the correct procedure to deal with a TCAS RA during intercept?
What is the correct procedure to follow if someone is intercepting you (or you are the F-16 driver intercepting someone else, for that matter) and you (or they, if you're the interceptor) get a TCAS RA in the middle of it all?  I'm thinking to cases like this situation where an interceptor may interpret the interceptee's TCAS-triggered maneuvers as signs of noncompliance with the intercept...yet the interceptor may not know just what prompted TCAS to issue the interceptee the RA, and the interceptee must comply lest it is an interloping third party on a collision course!

I rather doubt that F-16s have TCAS.  And, when he wants to avoid showing up on yours, he probably knows to turn off his Mode 3 (or perhaps just his Mode C, to avoid an RA) in order to accomplish that.

@RalphJ -- I'd expect the interceptor to strangle their squawk, yes -- but that didn't happen in the Tabatinga case, and there's also the question of what happens if a third party flies into the middle of things and triggers a TCAS RA from the interceptee.

That case was about 4 years ago, and lessons are learned from such things.  As to any 3rd-party-induced RA, that's got to be a pretty remote possibility, an RA and an intercept both happening at the same time??? Short answer, follow your procedures for the RA, and if you think you are being intercepted, get in touch with ATC ASAP.

Remote -- but not impossible, especially if you're performing an intercept in a busy Bravo...and yes, you have most of the pieces of an answer here, although I wonder if our resident Rhino Driver has something to say about this topic from the interceptor's POV.

@RalphJ: Why wouldn't an F-16 have TCAS?  I would imagine that the Air Force would rather not lose a multimillion-dollar fighter jet to a collision if it could avoid it...

@Sean -- getting civil avionics boxes to play nice with tactical jet systems is a non-trivial problems -- the tactical jets don't exactly follow the ARINC standards on that stuff all that closely, AIUI :P

The correct procedure for the interceptee would to follow the TCAS Resolution Advisory as they have generally no idea that they are being intercepted.
The interceptor must ensure that no TCAS RA is generated by switching his transponder to the appropriate mode. This has been standard procedure since the introduction of TCAS.
TCAS works by interrogating other aircraft's transponders. These interrogations are done in both Mode 3/A and Mode C as well as in Mode S (UF-0). To ensure that no TCAS RA is generated the interceptor's transponder must not reply to Mode C and Mode S interrogations from the interceptee. Since the source of Mode C interrogations is not encoded in the interrogation, the transponder must not reply to any of these interrogations including those from radar.
Technically a transponder could be designed to selectively not reply to Mode S interrogations from the interceptee only, however I think it is common to switch off TCAS Mode-S replies, or even switch  off Mode S entirely.

Indeed, in any type of military formation only one aircraft has an active squawk/active TA/RA squawk for just this purpose. Since the interception is an intention formation for all practical purposes, the same thing would happen

This is inconsistent with the general requirement that all aircraft squawk in Mode 3/Mode C while in a Mode C veil and in certain other situations (DC SFRZ, R-zones etc). U.S. military aircraft performing an intercept within FAA airspace will squawk 0000; I don't know if TCAS systems treat that code any differently.

@KeithS TCAS doesn't treat A0000 differently as far as I know. But if the aircraft is only replying on Mode A and not on Mode C there will not be a TCAS RA since that requires altitude (and hence Mode C or Mode S)

But you still have the legal requirement to operate in Mode C while in a veil, which exist around any Class B airport which are in most major cities including DC. So as the interceptor vectored to investigate an SFRZ violation, do you squawk Mode C and risk a TCAS RA by the interceptee indicating noncompliance, or do you switch to Mode A and handle IFR separation by alternate means (VFR, reporting altitude to ATC vocally)?

@KeithS -- never mind the possibility of having to do the intercept in actual IMC!

@UnrecognizedFallingObject, interception is done with interceptors and all of those have radars they can use to "see" and avoid. Modern ones also have some (encrypted and classified) telemetry to report their position to the military controller and that can then coordinate with the civilian ATC (the exact procedure differs by country).

@JanHudec -- the interceptor will probably pick up the third party interloper on their radar, but the third party interloper will likely have no clue about the intercept going on unless ATC passes them traffic about it...


text:
Can an APU exhaust make a contrail in flight?
I saw this picture in a local newspaper.

My thoughts were, silly artists, that's the APU exhaust, it won't ever produce a trail like that during takeoff (or ever).
Of course, the picture is clearly photoshopped, as can be seen by the planes' outlines. But somebody didn't do the research :)
Just to be sure, were my thoughts correct?

Which paper? De Telegraaf? metro? AD? zucht...

those are obviously smoke trails and the aircrafts are performing an aerobatic display.

@DeltaLima Gelderlander

Not sure they meet minimum separation rules either...

@Federico chemtrails, the evidence is in!

APU may be used in flight as second source of power or bleed air if one of the engine systems is inoperative. If it is so used, it might leave a contrail the same way engines will, though as DeltaLima notes, it's exhaust is hotter and the air behind the fuselage is more turbulent, so the range of conditions where it will leave a contrail is narrower than for the engines.
And it is an exceptional situation, so it is extremely rare. Most of the time the APU is switched off once the engines are running and only switched on again after landing, because when engines are running, they provide everything the aircraft needs.
In B737NG (the lower aircraft in the picture), the APU can be used as bleed air source up to 17,000 ft. This is too low for normal cruise, but if bleed system fails, pilots will descend to this altitude and start APU to provide air-conditioning for the remainder of flight to a suitable nearby airport. It can also be used up to 41,000 ft as power source, so the aircraft may cruise with one generator inoperative and running APU.
In A320 (the upper aircraft in the picture), the APU can be used together with one bleed air system up to 20,000 ft and for power generation up to 41,000 ft. So again, the aircraft may cruise with one generator inoperative and running APU.
However:

Normally there would never be contrail behind APU and none behind engines unless both engines failed and APU was started as backup power source (US1549 did start APU when the engines failed, though it is unlikely there was contrail at that altitude).
Contrail does not start right behind the engines. The water in the exhaust needs some time to cool down and freeze before the contrail becomes visible.

You are right. Newspapers are rarely able to produce anything about aviation without letting their imagination fly.
First of all, the APU doesn't produce contrails. Second, contrails don't start at the exhaust but at a distance behind it.

The APU could produce contrail if it is operating at cruise level. It just rarely is.

@JanHudec very unlikely as it isn't exactly a high by-pass system, so the exhaust air is relatively hot. This causes the condensation to happen quite far behind the aircraft. Since the exhaust is in the turbulent wake of the fuselage, the exhaust air will get mixed very quickly with the ambient air so there will be hardly anything visible. Now of course there are circumstances (humid air, just above dew-point temperature) that might cause an APU contrail but probability is very low.

Straight jets produce contrails too (older fighter jets do)

A B-17 can produce contrails. http://www.aerospaceweb.org/question/atmosphere/contrails/b17-contrails.jpg

@DeltaLima ANY heat engine consuming hydrocarbons is going to produce contrails if the air the exhaust exits into is sufficiently cold enough or at a low enough pressure to be below the dewpoint.

Its photoshopped.  If you look closely enough you will notice that the contrail from the A320 is a scaled down version of the one from the B737.
Something tells me the image came from a hysterical chemtrails conspiracy website.


text:
What are the major drivers of cost/price differences between Cessna 172 and Piper Cherokee?
This may be closed as opinionated but I'm looking for answers with hard data if I can get them. The Cessna 172 and Piper Cherokee are touted as two of the most successful light aircraft designs in the world, and they are primary competitors, with Piper originally designing the PA-28 Cherokee to be a lower-cost version of the Comanche that would compete directly with the 172 of the day.
However, in today's new market, a 2015 Cessna costs about \$370k, while Piper Archers (the continuation of the PA-28 airframe after Piper's corporate restructure) made in the last 10 years or so are selling for about \$200k. Cessna also seems to hold more value; "classic" 172 airframes from the 70s and 80s hold about a \$100k value in classified ads, while flight-ready Cherokees from the same time can be had for \$30-40k and a few Cherokee airframes are being offered at prices I've only ever seen 150s sold for (though there's likely something wrong with those examples).
The aircraft, while different externally (the 172's overhead-wing versus the Cherokee's low-wing design is the biggest difference), seem to have many similar specs on paper; passenger count, horsepower variants (Piper's actually better here as the 180hp variant was most common compared to the standard 172 powerplant at 160hp), instrument capabilities, etc. Cockpit pictures indicate the interiors are comparable; a bit dated as you'd expect from aircraft made 30 years ago, but perfectly serviceable.
So the question is, where's the additional retained "street value" of the 172 coming from? Is the 172 more well-reputed for safety or reliability? Are its handling characteristics considered better for everyday flight (perhaps due to the parasol wing)? Is this a question of average age of airframes? Is it just the Cessna name (in other words, am I comparing a Toyota Camry to a BMW 5-series)?
Obviously some of this stuff will depend on a comparison of specific hulls, but there's a definite trend; the market in general seems to value the 172 more highly than the PA-28 with all other things being equal on paper. The question is why.

You're comparing apples & oranges. Try looking at a 2015 Piper Archer DX; it's ~$400k.

OK, so factory prices are closer than I first saw. Cessnas still seem to hold a better used value; prices through the 70s and 80s MYs show Cessnas holding about $60-100k value in good maintenance, while Cherokee 180s are more in the $30k-50k range in similar apparent condition, even with upgraded instrumentation. Since I'm browsing the market for planes closer to the price of my car versus my house, these are the ones I'm really interested in and curious about the higher retained value of the Cessnas.

There are a lot of factors here but in reality a 172 and a Cherokee are about the same price, lets look the reality.
First off a huge chunk of people rent instead of buy. In this regard at the two airports I have been flying out of have both the 172s and Cherokees (both of which I have rented) are the just about the same exact price (all prices in USD).

Cherokee Wet  : 108
VFR Only 172: 103
IFR 172 : 113

The used prices (how a lot of people buy planes like this) span the field and I would say are on par (as per my latest perusing of Trade-A-Plane). That being said you can get a beat up Cherokee 140 for in and around 20K and a beat up 172 for about the same price (at least from what I have seen listed). At the same time you can get a fully loaded Cherokee 180 or 235 if you go complex with dual GPS/COMS and other toys for the better part of 60-70K, same for the 172s.
Yes they are similar planes but the truth is that the price things actually trade hands for is often not disclosed and is almost always not the list price in terms of used stuff. There is a bit of preference as well, if you trained in a Piper you may be more inclined to stay in a Piper and likewise with the Cessnas.
As for safety, the 172 is a very safe plane by anyone's standards. Then again so is the Cherokee and it seems that the argument can go on forever but the numbers are the numbers (accidents per 100,000 hours of flight)

The 172: 6.17 overall accident rate.
The Cherokee: Cherokees came in at 6.86

In reality a small difference, but lets be honest they are about on par. For what its worth, the Cherokee numbers include the Arrow which is a retractable and suffers from the proverbial gear up landing as well as other things that make a retractable potentially fail more than a fixed gear plane. This may account for the difference in safety record (again, a very slight difference).
While similar on paper there are some mission profiles that allow the planes to serve different roles and may make the 172 more attractive in some cases. The high wing design allows for excellent downward view which makes the 172 a great plane for aerial surveying/photography/pipeline check ups and similar tasks. The 172 also sports two doors compared to the single one on the Cherokee. That being said some feel as though the 172 is a bit easier to get into for the less nimble, but I am young and nimble so I can't speak to this. There are also those who like the high wing design for landing on grass or other back country flying due to the increased ground clearance of the wing allowing you to avoid obstacles.

From an aircraft family standpoint the Cherokee 235 had a much better useful load than the highest power 172's. However even the 180 Cherokee has a better useful load than the 180 Cessna. When you are flying around a family this is worth considering.
Although Cessna had it to an extent, Piper has always had the selling point of "stepping up through the planes". The idea being that your mission (as time went on) would be to go faster, further, and higher. Subsequently you would need a bigger plane. They facilitated this by delivering a very constant platform in the, Cherokee, Arrow, Cherokee 6/Lance/Saratoga then up through twins with the Seminole and Seneca if you need to go there. While I'm not sure I have ever seen any real data on this, I do have a real world experience with a buddy of mine who has been flying for about 25 years.
In response to the question "Have you ever owned any other planes?"

Yeah, I had an Archer for a while, it was a great plane. But then I had a third kid so I bought a Saratoga.

Like it or not the Piper sales model there worked.
You could also forgo common sense and just buy a Mooney because everyone else put the tail on wrong.

An older Mooney, like an early '80s, doesn't look that far out of reach of mere mortals; biggest hurdle is they're too powerful to fly with a student or rec license.

You will only need to add on a complex and high performance endorsement which you can do all in one shot if you can get into an Arrow or something similar. You will need complex for a Cherokee 235 and most other planes that are anything above a trainer even with fixed gear. In the long run the 5 or 10 hours it may take to get the endorsement will gain you a lot of distance on your flights. Rec license is a whole different story (that I dont know much about)

From further reading on the subject, it seems like the recreational license is on the way out; a lot of flight schools don't even bother offering this level, as the SPL is easier to get (no medical, 20 hours total time), while the PPL is not much different than getting an RPL plus the endorsements to make it useful (B/C/D airspace, night, cross-country) and it has far fewer restrictions on equipment.


text:
Can a steady flow have stagnation points?
I am confused by this idea. If a flow is steady, its streamlines are unchanging. Taking the typical example of an airfoil, there is (at least) one streamline which will hit the leading edge of the airfoil and stagnate. Stagnation point is defined as:

a point in a flow field where the local velocity of the fluid is zero.

Now my question is, if the velocity here is zero, and fluid particles that pass through the streamline which results in a stagnation point have nonzero velocity upstream, where do these fluid particles go? Since they are on the streamline, they have to reach the stagnation point? Does this not defy the conservation of mass law?
Further, an online Fluid Mechanics course I have been following has in one lecture shown streamlines going away from the stagnation point. If the velocity at this point is precisely zero (and is unchanging since the flow is steady), then how can the fluid particles go in other directions?
I must be missing something, do enlighten me.

A common misunderstanding lies in the fact that air particles are not motionless when we say there is zero-velocity wind (or airflow), otherwise pressure would drop to zero. A stagnation point is an infinitely small volume in which the airflow velocity is zero. This is a mathematical definition model, not exactly the physical reality

"fluid particles that pass through the streamline " in a steady flow streamlines are trajectories and parcels cannot cross them.  A parcel between two streamlines in this case will always be between those two streamlines.

The idea of a stagnation point is an idealization. This point is infinitesimally small, and air particles flowing along a streamline which leads into it will slow down on their way. The closer they come to the stagnation point, the slower they flow, and in the end they never arrive at the stagnation point.
In reality, air molecules have a finite size, so they flow either above or below the stagnation point streamline. Even if one particle manages to arrive at the stagnation point and stays in place (which is not possible in theory), a small angle of attack change will wash it away in the next moment.
One streamline goes away from the rear stagnation point which is located in the trailing edge. This point is doubly idealized, because it requires inviscid flow to have a rear stagnation point. Again, molecules flowing along the wall of the airfoil will be slowed down by the stagnation point pressure the closer they are to the trailing edge. Since they arrive either above or below the rear stagnation point, they will accelerate once they have passed the point of highest pressure and move on above or below the streamline emanating from the rear stagnation point.
The concept of a stagnation point is really helpful for understanding flow phenomena, however. There is really a line (in 3D it is a plane) which separates air that will flow over the wing from that which will flow below it. This line changes with angle of attack, and the simple stall warning vane is based on this principle. Once the line ends below the vane, air will press it upwards, closing an electrical contact which activates a buzzer in the cockpit.

Stall warning vane (small metal thingy sticking out from the wing)

When you say rear stagnation point, do you mean stagnation points at the trailing edge of the airfoil?

Also, to confirm what I was asking. Then it is theoretically impossible for a steady flow to have a stagnation point right?

@midnightBlue: Yes, the rear stagnation point is at the trailing edge. Ideally. If your understanding of stagnation points is that they collect air molecules which will never escape them, then yes, those do not exist in practical life. They are practically impossible, but theoretically they are very much alive (but infinitesimally small).

Perfect! I think I begin to understand it slightly better now. Do you know what is the sensitivity of the warning vane? (a sudden temporary local change in angle of attack could trigger a stall warning?). Also, is the location and set angle of the warning vane an approximately best selection? (i.e. under certain different conditions, would the warning vane be better placed elsewhere with a different preset angle?)

@midnightBlue: The location and incidence is the result of testing. You want some margin, but the thing should not constantly go off. The sensitivity is quite good: A gust in slow flight can trigger it, but then you can be sure that the wing at this location is just a few degrees away from stalling, so you better speed up. In a good landing the warning will sound a few seconds before the wheels touch the ground.

A stagnation point in a flow does not defy the conservation of mass law. Peter Kämpf already explained that it is an idealization which helps to understand flow phenomena.
The fact that the velocity is zero at a point along a stream line does not mean that mass is collected at that point. You can compare it to a stop sign on a road. Cars arrive at the sign, stop and pull up again. As long as there is sufficient space between successive cars this can be a continuous process representing a steady flow.
Taking this analogy further, the stagnation point at the leading edge of a 2D profile is like a T junction on a one way road. Cars arriving at the T junction stop and continue with either a left or right turn and follow thier paths. Air molecules arrive at the stagnation point and continue from there either along the top or the bottom of the profile.

Velocity is an instantenous phenomenon. It exists at a point.(lets ignore Average velocity here, it isn't necessary that we discuss it for this question.) At the stagnation point, the velocity is 0. However, at a distance dx from the stagnation point, the fluid particle again gains momentum. It sort of is like a pendulum. At the highest point its velocity is zero, but after a differential time dt, it again regains its velocity and starts to move. You see fluid particles like a pendulum need a stimulus to do so, in the case of pendulum its gravity, for a fluid particle, it can be anything from a pressure force to inertia


text:
How do I track and visualize aircraft from home?
I have a R820 USB SDR and I am using ADS-B to track aircraft flying near my home.  I know I could also use ACARS to track nearby aircraft.  Are there any other aircraft signals I can monitor (doesn't necessarily have to give aircraft location, just info)?
Thanks!

Are you including VHF radio in "other signals"? Although listening to radio traffic isn't legal everywhere.

Yes, listening to pilot <-> ATC would be cool.  Do you know the freqs?

Radio comms is a large topic by itself. The aviation band is 108-137Mhz (navigation beacons and voice) and there are lots of frequencies in use depending on where you are and who's talking. You might want to research this and then come back with some specific questions; LiveATC would be a good place to get a general 'feel' for the radio traffic.

Radar and TCAS interrogations on 1030 MHz. You need a sample rate of at least 4 MS/s to be able to properly detect the phase shifts though.

Yes, there is other signals broadcasted by aircraft, although not by all. In the General Aviation sector, at least in Europe, an on-board device known as FLARM is being used mostly in Gliders, Helicopters, Single-Engine Piston aircraft and touring motor gliders.

FLARM is an electronic device to selectively alert pilots to potential collisions between aircraft. It is not formally an implementation of ADS-B, as it is optimised for the specific needs of small aircraft such as gliders, not for long-range communication or ATC interaction.

You can visualize these signals using some tools and you can also feed the data to sites like flightradar.

Related Answers to FLARM: What system is there in the UK to prevent collisions between small aircraft?
How does TCAS work?


text:
How to become a expert in Flight Operations Business? (Flight Dispatch, Weight and Balance etc.. )
I am working in IT industry relavent to airline operations (like flight dispatch related products).I would like to enhance my knowledge in Flight operations area.
What are the trainings (IATA/others) or courses  recommended for flight operations related business process?

A starting point insofar as weight and balance is concerned, would be to become thoroughly familiar with the FAA's Advisory Circular AC 120-27. You can pick up a PDF of it at https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC120-27E.pdf. As I remember, it doesn't cover computerized systems, but it's still good for a basic introduction. Weight and balance complexity can run from the very simple to the very complex depending on the aircraft and usage. If you want to play with an online w&b program, you can do so at wb.terryliittschwager.com.

It depends on where you are located and what country you are located in as to what kind of license will be beneficial to you. If you are in the United States, obtaining a FAA dispatch license is a common route to working in flight operations. There are community college programs to prepare you to take the exams. The benefit to the FAA license is it never expires. In other countries there are similar licenses you can obtain. I would be cautious to IATA programs mainly due to the price and many are geared towards existing professionals in the industry.

It would be a little lengthy, but taking a course to get a Dispatcher license would cover a wide range of those topics in depth.
One example from Jeppesen: here.
A Google search for course dispatcher license will give you other options.

Thank you for sharing information.  How about iata airline operations diploma..  Will that be good?


text:
Was the March 2019 Rafale jet ejection incident investigated?
This is one of the wildest stories I’ve ever heard of.  Then again, this is CNN’s disaster-a-minute-for-ratings kind of reporting.
https://www.cnn.com/travel/article/france-fighter-jet-ejection-scli-intl/index.html
A group of French co-workers organizes a ride for a 64 year old fellow worker in a Dassault Rafale jet fighter wherein the man panics during flight and pulls the ejection handle, bailing out mid flight and landing in a field near the German border.
I stare completely baffled at this as to

How these people managed to convince the French Air Force to let this man ride back seat in a jet fighter
without any kind of familiarization or ejection seat training and
Nobody noticed the guy’s level of discomfort during pre-flight taxi or even takeoff?

Was there any in-depth investigation to the incident and if so, what was determined to be the root cause(s)?

Things happen all the time man, It's just part of life.

Well he had enough training to know how to fire the seat eh?

I guess he just panicked and grabbed anything that he could in the cockpit to hold onto!  I never knew it was that easy to get a ride in a Rafale!

Well you have to reach down to a pair of handles at the bottom of the seat, pull in trigger levers, then pull the handles up to fire it.  It's definitely a deliberate action.

https://aviation.stackexchange.com/q/61499/1467

Here are a few key points translated from the media coverage on the official BEA report (PDF, in French):

The passenger was a 64 years old manager of a shipping company.
The French Air Force carries out such passenger flights occasionally.
The passenger wore a pulse watch which recorded between 136bpm and 142bpm prior to the flight, safe to say the man was under a lot of stress.
The report states that the situation produced significant peer pressure on the passenger: A significant number of colleagues from work, who had organized the event, were present.
In order to max the element of surprise, the passenger passed the required medical exam the same day. (As a consequence of this accident, there is now a 10 day mandatory delay between the exam and the actual flight.)
The physician issued the medical certificate under the condition that the flight must not exceed 3G. This information, however, did not reach the pilot due to what the report calls a "technical problem". (This type of aircraft reaches close to 4G during a normal takeoff.)
The passenger wasn't correctly equipped: The anti-G trousers as well as the chinstrap were too loose, the helmet visor folded up. The passenger lost his helmet mid air after the ejection.
The passenger ejected in a phase of the takeoff with negative G forces.

Most likely, the passenger was under an extraordinary amount of stress and tried to hold onto something in negative G, unfortunately the eject cord.
There's, however, another not so fun fact:
The pilot should have been ejected automatically along with the passenger. This mechanism failed! The pilot could have ejected himself nonetheless, but chose not to. It was due to this technical malfunction that the French Air Force didn't lose such an expensive machine.
(sources: Le Point, BFM TV)

Interesting, can you add a link to the BEA report?

I looked for it in the BEA database, but it is not (yet?) listed for 2019-03-20, the date of the accident.

Found it, added the link.

I don't like to edit, but it's worth noting the passenger was left to strap himself into the plane. Absolutely dire.

This really leaves the impression that the French military is run by children.

I wish I could read French! :(

In this case the pilot was not ejected because the link failed, but there should actually be a mechanism to disconnect it. I remember reading of similar accident long ago in the USA (I believe in that case the passenger was an army officer, but not a pilot) where also just the passenger ejected, but in that case the seats were deliberately configured so the pilot could eject both, but the passenger could not. It was a different aircraft type, so I am not sure the option is available on Rafale too though.

@JanHudec  It probably would have been impossible to do so, as the seats are an integral safety feature of the jet.  Most civilians who fly in military jets in the US eg a local reporter or celebrity goes for a ride in Blue Angels #7, etc. are given training briefs on ejection seat usage and safety, parachute landing, etc.  The back seat remains hot and can be activated by the passenger.

@CarloFelicione, I didn't say it does not remain hot. I said it can be deliberately set so that pulling the ejection handle does not eject the other as well (as it normally does in case the other crew member is incapacitated).

@JanHudec The "sélecteur de séquence" (sequence selector) controls whether both seats eject or not, it can be set to either TWO or SOLO. In this case, it was on position TWO, however, the line from the selector to the front seat pyro was disconnected, ripped out of its casing (see image on page 26 of the BEA PDF linked above).

Air Force (fixed)

the incident we're thinking about

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Lockerbie bombing - amount of debris (confusing numbers)
As mentioned on Pan Am Flight 103:

... In total 4 million pieces of wreckage were collected and registered on
  computer files. More than 10,000 pieces of debris were retrieved,
  tagged and entered into a computer tracking system.

This is a bit confusing, at least to me. So which one is the actual number? Or if both sentences are correct, what is the difference between them?

IMO most likely explanation is 2 edits from different times with different sources

Any idea which one is correct number? Tried to look thru official report but I can't seem to find the number in there. 10k seems low - perhaps this number describes some pieces that had a specific meaning for the investigation? Or specific type?

I believe both numbers are correct. There were millions of pieces of wreckage from the plane as well as the surrounding houses that were destroyed. Everything was collected and examined. Of those ~4 mil pieces of wreckage, something over 10000 pieces of debris were confirmed to be from the plane.

This actually makes sense CGCampbell, thanks. In other words then, investigators were able to identify 10k as plane parts but it doesn't mean these were all of them?

Pieces of wreckage implies a large 'section' of plane, whereas debris suggests miscellaneous smaller items and bits of metal etc.
I agree it's potentially confusing, however, and likely arises from two people editing the same section without adjusting the context.
I'd say that 4 substantial sections of plane were found, along with a large amount of smaller debris scattered around, totalling around 10,000 pieces.

They must have been feeding from two different sources then. I mean, the 4 million pieces is published on natgeo website (http://www.natgeotv.com/za/air-crash-investigation/lockerbie). But the Lockerbie Bombing wiki page (as opposed to pan am flight 103 wiki page) says 10,000 items. I'm surprised the number is not stated in the raport

@belinea, there are potentially dozens of ways to count debris... The question was about the specific sentence on Wikipedia.

I think you might have missed the "million" in the original quote

@JonStory Wikipedia said 4 million pieces of wreckage and 10,000 of debris, and you said wreckage are large pieces and debris is small. So they found 4 million large pieces and 10,000 small ones?


text:
Can thrust reversers possibly collapse due to strong crosswinds?
I recently watched a YouTube video titled "Dangerous Crosswind Landings during a Storm at Düsseldorftitled" and witnessed at the 2:06 mark, a twin engine commercial jet aircraft landing and noticed only one thrust reverser was deployed.
I am looking for a knowledgeable answer, asking if this was a malfunction.

It is impossible to tell from the video what the reason or cause was, this is all speculation.
Most likely it was a malfunction of the thrust reverser. There is a good change that the crew was aware of the problem before take-off; the Airbus A330 may be dispatched with one or even both thrust reversers unserviceable according to the minimum equipment list (MEL)
Another possibility is that the crew accidentally only activated one thrust reverser. Given the firm landing (as it should be given the strong crosswind) they might have erred in the heat of the moment.
It is unlikely that the decision to use only one reverser was on purpose if two reverser were serviceable. On the ground the aircraft has a tendency to weathercock into the wind because of the vertical tail. Selecting only the windward thrust reverser aggravates that tendency.

What about selecting only the leeward thrust reverser?

Selecting only the leeward TR would tend to counteract the weathervaning force of the crosswind, but not in a manner that the pilot could precisely control. If you did that, then you'd be using the rudder to maintain directional control with two asymetrical, and variable, forces also at work.  Standard procedure is to use all available TR's, with the caveat that if they (or, "it") interfere with directional control, reduce the reverse thrust as needed.  Since the response time of reverse thrust isn't immediate, modulating it to counter effects of a crosswind isn't generally recommended.


text:
What is the average number of daily flights for all types of aircraft in use worldwide?
I'd like to know how many flights occur on average every day worldwide. This includes all commercial passenger and cargo aircraft, all non-commercial aircraft, private aircraft and military aircraft. A flight consists of a planned take-off and landing.

A training aircraft takes off, goes to a small airport, maybe even an uncontrolled field, does a few touch & go landings, returns to where it started, lands, taxis to parking, and shuts down. One flight? Or several since the touch & go landings each include "a takeoff" and "a landing"?  A control tower will count each T&G as two events (they controlled the landing and the takeoff); nobody at an uncontrolled field records anything. Some aircraft won't even have the count of T&G's recorded anywhere in the maintenance logs. A pilot might, but isn't required to, record the count in his logbook.

The question is perfectly specific, but it would be incredibly hard to get anything even remotely close to an exact number unless every controlled field in the world reports their traffic counts back to some central database. Approximations are probably available somewhere out there.

For airlines: 100.000 flights per day (source: What are the statistical probabilities of commercial aircraft accidents?) For GA, there are no equivalent worldwide statistics.

As mentioned above there are a significant number of aircraft that have no way of being recorded. However, we can make an educated guess. According to the FAA, they support; "Every day, the FAA's Air Traffic Organization (ATO) provides service to more than 42,000 flights" - https://www.faa.gov/air_traffic/by_the_numbers/. These numbers would be aircraft with flight plans, controlled airport landing & takeoffs, flight forwarding, vectoring, etc.
Probably (a guess) at least as many general aviation aircraft do training, VFR flights, helicopters (i.e. ranch cattle herding), balloons, scientific, skiing, hunting, sightseeing, gliders, ultralights and maintenance flights without filing a flight plan or interacting with ATC.
So that puts the US at about 80,000 flights per day. The US is approximately 5% of the world population and if we use that as a basis for calculating flights, then there are about 1.6 to perhaps 2 billion flights per day, including all forms of aircraft.
This is interesting - we might at first conclude 1/4 of the earths population is flying on any given day. This would be a wrong assumption because many passengers embark on several flights to reach a destination each day. For an example, a sightseeing cross country trip might include 3 or 4 stops that include re-fueling and eating, or restroom breaks. But, this is still a staggering number of people flying each day. Conservatively, perhaps 1 out of every 50 to 100 people are traveling by air each day!!!

While I like the analytical approach you took, I don't think that the leap from "the US is approximately 5% of the world population" to multiplying 80,000 by 20 is correct. The US has an outsize percentage of the world's aviation because it has such a well-developed aviation sector compared to almost every other country. Unfortunately, the data is hard to find, but the factor of 20 is almost certainly overstating things.

I considered that before answering but came to the conclusion that  undeveloped countries are more apt to use bush aircraft than the US. In the end, I think it would be near equal. The number of flights being equal, the US would use jets, while other countries would use recip or turbo prop. I arbitrarily used US statistics because they were available to me.


text:
How did the term PAN PAN originate?
Where did the phrase "pan-pan", used when declaring an emergency, originate?

Wikipedia has an un-cited reference to the etymology being

As with mayday (from venez m'aider, "come help me"), the urgency
  signal pan-pan derives from French. In French, a panne ([pan], "pahn")
  is a breakdown, such as a mechanical failure. In English, it is also
  sometimes[vague] pronounced as /pæn/ ("pan").

This article supports that history (and has many of the other emergency phrase histories as well).

The last link sends to an ads page now.


text:
Is there a requirement for self-locking / positive locking electrical connectors for aircraft systems?
Is there any FAA or EASA regulation that dictates the use of locking electrical connectors for certain systems on an aircraft?
Self-locking (ratchet mechanism, bayonet) and positive locking (wire locked) connectors are used on most systems.
But there are also push-pull connectors mainly used on headsets or quick disconnect batteries.
Is there a regulation that prohibits the use of not self-locking or positive locked electrical connectors for certain systems even though they fulfill exactly the same mechaninal specifications concerning vibration, shock and durability?

Vibration, shock, and durability aren't the only reasons to use self-locking connectors. A mechanism to make it pop out on its own if not properly mated helps alert you if you haven't plugged it in correctly; a good wire lock can't be fastened unless the connector is fast. Either helps prevent disconnections caused by careless maintenance, which is a more common cause than mechanical failure due to shock or vibration.

@DanHulme -- then again, many disconnections due to careless maintenance are due to the careless guy unplugging the connector and then forgetting it was ever unplugged, thus not remembering to plug it back in :)

EDIT: Note that the original question, which this answer primarily addresses, did not specify electrical connectors.
The FAA does provide regulations which at least partially cover this subject, though the guidance is rather broad.
The airworthiness standards in 14 CFR 23.607 for normal, utility, acrobatic, and commuter category airplanes stipulate that:

(a) Each removable fastener must incorporate two retaining devices if the loss of such fastener would preclude continued safe flight and landing.
(b) Fasteners and their locking devices must not be adversely affected by the environmental conditions associated with the particular installation.
(c) No self-locking nut may be used on any bolt subject to rotation in operation unless a non-friction locking device is used in addition to the self-locking device.

Likewise, the  airworthiness standards in 14 CFR 25.607 for transport category airplanes stipulate that:

(a) Each removable bolt, screw, nut, pin, or other removable fastener must incorporate two separate locking devices if—
(1) Its loss could preclude continued flight and landing within the design limitations of the airplane using normal pilot skill and strength; or
(2) Its loss could result in reduction in pitch, yaw, or roll control capability or response below that required by Subpart B of this chapter.
(b) The fasteners specified in paragraph (a) of this section and their locking devices may not be adversely affected by the environmental conditions associated with the particular installation.
(c) No self-locking nut may be used on any bolt subject to rotation in operation unless a nonfriction locking device is used in addition to the self-locking device.

As you can see, these regulations do prohibit certain types of locking fasteners in certain circumstances. For example, a nylon self-locking nut is not acceptable to fasten the exhaust manifold to the cylinder head; an all-metal self-locking nut would be acceptable, however.
The greater concern for a system such as a headset may be that to install it with fastening hardware—screws for example—would be to make it an installed part of the aircraft. The installation would then be subject to the various regulations of an installation: a log book entry, weight and balance considerations, etc.
Additionally, AC 43-13-1B, Ch 7, provides non-regulatory guidance regarding use of fasteners, including the use of locking and safety mechanisms. I will note that this guidance is superseded by manufacturer maintenance data for most modern aircraft.

Thanks for the interesting source of information although the cited paragraphs are not explicitly related to electrical connectors.

@robsn In using the word "connector" in your question, do you mean specifically "electrical connector"? In US aviation parlance, the word connector can be used of many different types of hardware and structures, for example seat belt fittings.

@robsn Additionally, the cited paragraphs from parts 23 and 25 do address fasteners for electrical terminals, where applicable.

Yes, the question is about electrical connectors only. I'm sorry for the confusion, I'm not a native english speaker. I'll edit the question accordingly.

authors: Михаил Черняев


text:
Are there aircraft exemptions for CPDLC in the NAT area?
Another question lists the aircraft that are exempt from the EASA CPDLC rule.
Is there a similar exemption for aircraft operating in the North Atlantic?

I think there is no exemption possible. But not all tracks and levels are covered by the NAT CPDLC rules.

@DeltaLima Not all tracks yet, but next year they will be and all NAT airspace in 2017.

Looks like it indeed. You'll have to fly below FL350 or above FL390. Climbing through the FL350-390 block seems to be possible for unequipped aircraft.

Or fly through the ADS-B corridor from Canada to Iceland. There is no datalink mandate in surveillance airspace  apparently.

There will be no exemptions for the CPDLC rule in the North Atlantic Track area.
The Datalink Mandate will be implemented:

starting 5 February 2015: FL350 to FL390 (inclusive) on all tracks within
the NAT Organized Track System (OTS)
starting 7 December 2017: FL350 to FL390 (inclusive) throughout the ICAO
NAT region.
starting 30 January 2020: FL290 and above throughout the ICAO NAT region.

ATS Surveillance Airspace, airspace north of 80°N and the New York FIR are excluded from the mandate.
This means that aircraft equipped with a suitable transponder / ADS-B transmitter will be able to cross using the Canada - Greenland - Iceland ADS-B corridor without having to carry FANS 1/A equipment.


text:
What is the speed limit in European airspace?
In Did this aircraft illegally exceed 250kts below 10,000ft? it was mentioned that unlike here in the US, EASA does not have a 250 kt. speed limit below 10,000 ft.
So does this mean that are we allowed to go as fast as we want?

Mach 8? ;-)
What is the maximum indicated airspeed specified by EASA when operating in the European Union?

The answer in the linked question says it's up to the member states. I know for a fact that there's a 250 kts restriction below FL100 in Germany unless the aircraft needs to go faster due to its design. I assume they're referring to fighter jets and spacecraft... Also, love the picture :)

This is handled by the member nation's AIP, not the EASA. In Germany you have restrictions to maintain 250kts IAS below FL100 in all airspace except class C and supersonic over land is not allowed apart for military traffic in certain areas.

@SentryRaven Does EASA specify any speed limits (other than no supersonic over land)?

Doubtful, as EASA has no say in that. This is all national jurisdiction --> AIPs. Check @DeltaLima answer to your question.

It does have a say in that. All EASA directives automatically become legislation. It just chose to put speed limitations into SERA Annex 6, see SERA.6001 'a' through 'f'

I am confused-- nothing in the attached picture resembles an aircraft.

Airspace regulations do not fall under the authority of EASA. Each country has their own set of rules published under supervision of their own National Aviation Authority in the AIP.
The origin of the 250 knots IAS limitation below 10000ft can be found in the ICAO airspace classes definitions. The ICAO airspace definitions include a 250 knots IAS limitation below 10000ft / FL100 in:

Class C: VFR
Class D, E, F and G: both VFR and IFR

That means that in class A and B there is no speed limitation below 10000ft / FL100. In class C, IFR has no speed limitation, but VFR has. In Europe you will find a lot of class A, B and C airspace below 10000ft. In the US, there is no class A airspace below 18000' feet. There is class B and C airspace below 10000ft, but the FAA basically put a blanket speed limitation of 250knots below 10000ft, even inside class B and C airspace.
Also note that ICAO airspace classes are recommendations, countries have the authority to deviate from the ICAO recommendations as long as they publish these deviations.
Section ENR (En-route) 1.4 of the AIP contains the ATS Airspace Classifications and states includes the deviations from the ICAO airspace definition.

Seems complicated if you have a flight across Europe....  EASA was supposed to solve some of this confusion wasn't it??  (+1 though)

The airspace structure in Europe is indeed quite complex. For VFR it is quite difficult, for IFR not so.

As some other people already mentioned in the comments, EASA DOES have a say in it. And indeed it's all written in "SERA.6001 Classification of airspaces"
Link here for the Easy Access Rules doc https://www.easa.europa.eu/document-library/easy-access-rules/easy-access-rules-standardised-european-rules-air-sera
Summary:
For IFR flights

Class A, B and C airspace don't have any speed limit
Speed is limited to 250 kts IAS below 10000ft in all other classes of airspace

For VFR flights:

Class B doesn't have any speed limit
Speed is limited to 250 kts IAS below 10000ft in all other classes of airspace

Every member country can have more restrictive rules but usually they all stick to it to make it more simple (unless stated in their AIP).
By the way, all students are now trained using EASA material so it's relatively uncommon to have significant deviations from it.